1. Field of the Invention
The present invention relates to cellular communication, and more specifically to a method and apparatus for accurately detecting and reporting cellular base station antenna conditions.
2. Relevant Background
One of the most common sources of cellular communication problems is antenna and feeder cable failure at a cellular base station. A base station is typically exposed to many harsh environmental conditions which take their toll on the antenna and/or antenna feeder cable over time. Some of the more common causes of antenna and cable failure include corrosion from moisture, fatigue due to temperature changes, and stress from wind, hail, and lightning strikes. In addition, some gun owners are known to prefer base stations over conventional bottle and can targets when target practicing. These factors can lead to a slow degradation in antenna and/or feeder cable performance over time, or may cause complete failure all at once. In either case, a method for early detection of antenna and cable problems is critical in providing reliable cellular communication services.
A known method of detecting base station antenna and/or feeder cable failure is by monitoring cellular traffic at base stations. A sharp drop in traffic is a good indication that a base station failure has occurred. When such a drop in traffic occurs, a technician can be sent to the base station to investigate possible antenna and cable problems. This method, however, is not helpful when a base station failure occurs in low cellular traffic locations, such as rural areas. In these areas, a lack of cellular activity may seem normal, and an antenna and/or cable failure may therefore not be noticed for long periods of time. Moreover, a slow degradation in antenna or cable performance may be difficult to identify by monitoring cellular traffic levels. A gradual decrease in cellular traffic can be caused by many factors, and may be impossible to correlate to a degradation in antenna or cable performance. What is therefore needed is a method to alert a cellular service provider of base station antenna and cable problems in low cellular traffic regions. What is also needed a method to accurately detect a gradual degradation in antenna and cable performance over time.
Another known method of detecting base station antenna and feeder cable problems is to routinely and periodically test the condition of base stations by a technician. For example, a technician may visit a base station every six months and perform various antenna and cable tests in an effort to determine the base station""s operating condition. Among these tests, perhaps the most important measurements taken are a return loss reading of the antenna (also referred to as the reflection coefficient or standing wave ratio) and a cable loss reading of the antenna feeder cable.
Return loss occurs when a portion of a signal being passed to an antenna is reflected back to the source. Return loss can be measured as the difference between amount of original signal input to an antenna and the amount of signal reflected back by the antenna. A large amount of reflected signal is undesirable since the amount of signal ultimately transmitted by the antenna is lower, thereby decreases the antenna""s effectiveness. Return loss is generally caused by impedance mismatching between the antenna and the signal source.
Typically, the input impedance of an antenna is matched to the output impedance of the signal source during antenna installation, thus minimizing any return loss. When an antenna is damaged however, its impedance changes, causing antenna and source impedances to become mismatched. As a result, the return loss of the antenna increases. Thus, measuring return loss is a method of determining if any antenna damage has occurred.
Cable loss describes the drop in signal strength across a cable, and can be measured as the difference between amount of original signal input to a cable and the amount of signal output by the cable. The difference between these two values is the cable loss of feeder cable. Several factors contribute to cable loss, including the cable""s dimensions, the ambient temperature, and the frequency of the signal input to the cable. In general, damaged antenna feeder cables have higher cable loss, and deliver weaker signals to the antenna. Therefore, measuring cable loss is helpful in determining whether damage to an antenna feeder cable exists.
A shortcoming of sending a technician to perform periodic antenna diagnostic measurements is that an antenna and/or cable failure may potentially go undetected for long periods of time. Increasing the number of technician visits can help detect base station antenna and cable problems in a more timely basis, but this also increases the cost of maintaining the base station dramatically. In addition, manually measuring antenna return loss and cable loss can be a dangerous undertaking. Often times, cellular antennas are mounted on high towers or other difficult to reach peaks, and technicians must climb to these antenna locations in order to accurately measure return loss and cable loss.
What is therefore needed is a method to detect antenna and cable damage quickly and accurately in a safe and efficient manner which overcomes the shortcomings of the prior art.
Briefly stated, the present invention involves base station having an antenna coupled to a cabinet through a feeder cable. The base station includes an antenna forward signal power detector adapted to output an antenna forward power signal for signals traveling from the cabinet to the antenna. An antenna reflected signal power detector is adapted to output an antenna reflected power signal for signals traveling from the antenna to the cabinet. An antenna return loss signal is provided which is proportional to a difference between the antenna forward power signal and the antenna reflected power signal. A return loss oscillator is configured to modulate the antenna return loss signal, and the oscillator is coupled to the feeder cable. A cabinet circuit is coupled to the feeder cable. The cabinet circuit is configured to restore said antenna return loss signal. The base station may further include a cable loss oscillator configured to modulate the antenna forward power signal. The cable loss oscillator is also coupled to the feeder cable.
Another aspect of the present invention is a base station having an antenna coupled to a cabinet through a feeder cable. An antenna forward signal power detector is adapted to output an antenna forward power signal for signals traveling from the cabinet to the antenna. An ending cable loss signal is generated which is proportional to a difference between the antenna forward power signal and an antenna voltage reference. A cable loss oscillator is configured to modulate the ending cable loss signal. The oscillator coupled to the feeder cable. A cabinet circuit, also coupled to the feeder cable, is configured to restore said ending cable loss signal. The cabinet circuit may further include a cabinet forward signal power detector adapted to output a beginning cable loss signal for signals traveling from the cabinet to the antenna, and a cable loss status signal proportional to a difference between the beginning cable loss signal and the ending cable loss signal.
Another aspect of the prevent invention is a base station which contains an antenna coupled to a cabinet through a feeder cable. The base station includes a first RF coupler configured to detect forward signals traveling from the cabinet to the antenna. A second RF coupler is configured to detect reflected signals traveling from the antenna to the cabinet. A first power detector is coupled to the first RF coupler and is configured to provide a forward power signal of the forward signals. A second power detector is coupled to the second RF coupler and is configured to provide a reflected power signal of the reflected signals. A differential amplifier is coupled to the forward power signal at a non-inverting input and is coupled to the reflected power signal at an inverting input. The differential amplifier provides a detector tuning signal which is an amplified difference between the forward power signal and the reflected power signal. A voltage controlled oscillator is coupled to the detector tuning signal at a detector oscillator input and is coupled to the feeder cable at a detector oscillator output signal.
The base station may further include a phase detector having a first phase detector input, a second phase detector input, and phase detector output. The first phase detector input is coupled to the feeder cable. The phase detector output provides a recovery tuning signal proportional to a phase difference between the first phase detector input and the second phase detector input. A second-voltage controlled oscillator is coupled to the recovery tuning signal at the oscillator""s input and is coupled to the second phase detector input at the oscillator""s output. The phase detector and the second voltage controlled oscillator form a phase locked loop such that the recovery tuning signal tracks the detector tuning signal.
Another aspect of the present invention is a method for providing an antenna condition signal in a base station having an antenna coupled to a cabinet by an antenna feeder cable. The method includes the steps of: detecting the power level of a forward signal traveling to the antenna; detecting the power level of a reflected signal traveling from the antenna; providing a detector signal proportional to a difference between the power level of the forward signal and the power level of the reflected signal; tuning a carrier signal to a frequency corresponding to the detector signal; and inserting the carrier signal into the antenna feeder cable.
In addition, the method may further include the steps of extracting the carrier signal from the antenna feeder cable; providing a recovered signal proportional to a difference in phase between the extracted carrier signal and a reference signal; and tuning the reference signal to a frequency corresponding to the recovered signal.
Another embodiment of the present invention is a method for providing an cable condition signal of a feeder cable. The method includes the steps of: detecting a first power level of a forward signal proximate the antenna end of the feeder cable; forming a first cable loss signal responsive to a difference between said first power level of said forward signal and a first voltage reference; tuning a carrier signal to a frequency responsive to said first cable loss signal; and applying said carrier signal into said feeder cable.
In addition, the method may further include the steps of: extracting said carrier signal from said feeder cable; recovering said first cable loss signal from said carrier signal; detecting a second power level of said forward signal proximate the cabinet end of the feeder cable; forming a second cable loss signal responsive to a difference between said second power level of said forward signal and a second voltage reference; and forming a feeder cable status signal proportional to a difference between said second cable loss signal and said recovered first cable loss signal.